fix(tci): derive the sample width instead of trusting the format number

A real SunSDR answers format=3 where this expected 0 — and 0 was read
from the documentation, which is exactly the kind of detail a memory of a
document gets wrong. The stream was refused outright: 'format=3, expected
float32', zero frames, silence.

Swapping one magic number for another would only move the guess, so the
width is now MEASURED: the header says how many samples the payload
holds, and dividing gives the bytes per sample. Four is float32, two is
16-bit PCM scaled to the same -1..1 the rest of the audio path uses, and
anything else is reported rather than mangled. That stays true whatever
number the format field carries on the next firmware.

internal/cat/tci_audio.go

@@ -83,6 +83,9 @@ type tciAudio struct {
 	peakAt  time.Time
 	probe   int
 	lastErr string
+	// widthLogged keeps the one-line note about the sample width to once a
+	// session — it is a fact about the radio, not an event.
+	widthLogged bool
 
 	// OnSamples receives decoded MONO samples (the two channels averaged) at
 	// the negotiated rate. Mono because everything downstream — the QSO
@@ -176,24 +179,63 @@ func (t *TCI) handleBinary(data []byte) {
 	if stype != tciStreamRXAudio {
 		return // IQ, TX audio echo, chrono: not this file's business yet
 	}
-	if format != 0 || codec != 0 {
-		t.audioErr(fmt.Sprintf("stream is format=%d codec=%d, expected float32 uncompressed", format, codec))
+	if codec != 0 {
+		t.audioErr(fmt.Sprintf("stream is codec=%d, and nothing here decodes a compressed stream", codec))
 		return
 	}
 
+	// The FORMAT number is decided by measurement, not by the number itself.
+	//
+	// A real SunSDR answered format=3, where the code expected 0 — and 0 was a
+	// guess from reading the documentation, which is exactly the kind of detail
+	// a memory of a document gets wrong. Rather than swap one magic number for
+	// another, the sample width is derived from what arrived: the header says
+	// how many samples the payload holds, so the bytes per sample follow from
+	// dividing. That is true whatever number the format field carries, on this
+	// firmware and the next.
 	payload := data[tciHeaderBytes:]
-	n := len(payload) / 4
+	if len(payload) == 0 || length <= 0 {
+		return
+	}
+	width := len(payload) / length
+	var n int
+	switch width {
+	case 4:
+		n = len(payload) / 4 // float32
+	case 2:
+		n = len(payload) / 2 // 16-bit PCM
+	default:
+		t.audioErr(fmt.Sprintf("frame carries %d bytes for %d samples (format=%d) — not a width this reads",
+			len(payload), length, format))
+		return
+	}
 	if n == 0 {
 		return
 	}
+	// Under the lock like the rest of the counters: the reader is the only
+	// writer today, but a fact about the radio that is read from another
+	// goroutine has no business being the one field left unguarded.
+	t.audio.mu.Lock()
+	first := !t.audio.widthLogged
+	t.audio.widthLogged = true
+	t.audio.mu.Unlock()
+	if first {
+		debugLog.Printf("TCI: audio is %d bytes per sample at %d Hz (format field says %d)", width, rate, format)
+	}
 	// Stereo interleaved → mono. Both channels of a receiver carry the same
 	// audio, and everything downstream works on one.
 	mono := make([]float32, 0, n/2+1)
 	var peak float64
+	sample := func(i int) float32 {
+		if width == 2 {
+			// 16-bit PCM, scaled to the same -1…1 the rest of the audio path
+			// works in, so a change of format cannot change what a level means.
+			return float32(int16(le.Uint16(payload[i*2:]))) / 32768
+		}
+		return math.Float32frombits(le.Uint32(payload[i*4:]))
+	}
 	for i := 0; i+1 < n; i += 2 {
-		l := math.Float32frombits(le.Uint32(payload[i*4:]))
-		r := math.Float32frombits(le.Uint32(payload[(i+1)*4:]))
-		v := (l + r) / 2
+		v := (sample(i) + sample(i+1)) / 2
 		if a := math.Abs(float64(v)); a > peak {
 			peak = a
 		}